You can get 'unlimited' drinks packages at functions in Australia on a per head basis - it's very common.
At my wedding the food was (about) $45 per head and drinks were (about) $35 per head.
Taking into account that the venue gets the drinks for far less than retail pricing, plus averaging the number of drinks consumed by an average person generally works out. Those who go hell for leather are usually balanced by the non-drinkers and those driving who consume less than the 'average' amount.
The bar tab doesn't run out but the staff are still obligated to cease supply to any individual who is visibly intoxicated under our responsible service of alcohol laws.